1. Field of the Invention
The present invention relates to a piezoelectric speaker for acoustic equipment.
2. Description of the Background Art
Piezoelectric speakers are known as small-sized, low-current-driven acoustic components using a piezoelectric element as an electric acoustic conversion element, and are used as an acoustic output device for small-sized electric equipment. Generally speaking, the piezoelectric speaker has a structure such as that of a metal diaphragm pasted with a piezoelectric element having an electrode such as a silver film. An alternating voltage applied to both surfaces of the piezoelectric element deforms the piezoelectric element to vibrate the diaphragm, thereby producing sound.
In conventional piezoelectric speakers, as disclosed in Japanese Patent Laid-Open Publication 2001-16692, for example, the diaphragm is supported so as to vibrate linearly, thereby flattening frequency characteristics. Therefore, in general, the diaphragm of the piezoelectric speaker is made solely of 42 stainless alloy (42Ni—Fe: hereinafter referred to as 42 alloy), because the 42 alloy has a coefficient of thermal expansion close to that of a PZT (lead zirconate titanate) piezoelectric material.
Here, the lighter in weight the diaphragm of the piezoelectric speaker, the better the sound pressure level per unit energy. Therefore, for piezoelectric speakers incorporated in portable terminal devices requiring long battery life and low voltage drive, a reduction in weight of the diaphragm is crucial for achieving better acoustic features.
The diaphragm of the piezoelectric speaker should also have appropriate stiffness. When the piezoelectric element has a thickness of approximately 50 μm (micrometer), the thickness of the diaphragm made of 42 alloy is in a range of approximately 50 to 100 μm. If the diaphragm is thinner than the above range, the stiffness of the diaphragm is decreased, causing difficulties in stably supporting the piezoelectric element and sufficiently converting shape distortion of the piezoelectric element into vibration. If the diaphragm is thicker than the above range, the stiffness thereof is extremely increased. Therefore, vibration of the diaphragm cannot be obtained, leading to a reduction in sound pressure level. For this reason, the diaphragm of the conventional piezoelectric speaker cannot be made extremely thin for weight reduction because of the requirement of appropriate stiffness to maintain acoustic features. Also, the diaphragm of the conventional piezoelectric speaker is made solely of metal material (42 alloy) having a high density in accordance with the coefficient of thermal expansion of the piezoelectric material. Therefore, it has been difficult to achieve a reduction in weight of the diaphragm with different materials. It has also been difficult to achieve an improvement in sound pressure level per unit energy that would have been brought by weight reduction.